This invention pertains to electrochemical cells and more particularly is concerned with liquid cathode cells.
Primary electrochemical cells are utilized for a wide variety of applications and are commonly available in a large assortment of sizes and shapes, including cylindrical. A cylindrical cell is disclosed in U.S. Pat. No. 4,376,811 in the name of Franz Goebel and includes a lithium anode, a carbon cathode current collector structure, and a porous separator interposed between the lithium anode and the carbon cathode current collector structure.
The carbon cathode current collector structure is physically pressed against the interior wall of a metal housing and is arranged concentrically within the housing with the separator and lithium anode. The assembly of the lithium anode, carbon cathode current collector structure and separator is exposed to an electrolytic solution including a reducible liquid cathode solvent and an electrolyte solute dissolved in the reducible cathode solvent. Suitable materials for the reducible cathode solvent and the electrolyte solute are thionyl chloride and lithium tetrachloroaluminate, respectively.
In the normal discharge of such a cell, the reducible cathode solvent is catalytically reduced at the surface of the carbon cathode current collector structure. This catalytic reduction results in the formation of a variety of reaction products within the cell and physically consumes available carbon sites, thionyl chloride and lithium until one of these components is depleted. The life span of the cell is to a large degree dictated by the amount of lithium and thionyl chloride initially present in the cell and the rate at which the thionyl chloride and lithium are depleted by electrochemical action with the cell.
A further, and undesirable, reduction of the thionyl chloride also takes place at those metallic portions and surfaces of the cell at the same electrical potential as the lithium anode. This latter reduction of the thionyl chloride, which may take place during storage of the cell prior to normal discharge of the cell, is a parasitic self-discharge reaction and can lead to an undesirable capacity loss and a premature termination of the rated life span of the cell.
To prevent parasitic discharge, it is known to keep the electrolyte separate from the other cell components during storage and until activation. Cells having this feature are known as reserve cells. In one arrangement, a glass ampule containing a ready-to-use electrolyte is centrally located within the cell.
As is well known, a battery includes a plurality of cells arranged in series to provide a voltage greater than that possible from a single cell alone.
Furthermore, it is known that the anode and cathode current collector structure of adjacent cells may be on opposite sides of a conductive carrier plate or substrate. The assembly is called a bipolar plate. Batteries have been made with a multiplicity of flat bipolar plates arranged in a linear stack with the end of the stack terminated by plates carrying at the other end, an anode at one end and a cathode current collector structure. An insulating separator made of a thin porous material such as glass paper is interposed between facing anode and cathode current collector structures. A quantity of electrolyte solution is carried between the plates. The elements and electrolyte between adjacent carrier form a cell. Adjacent cells are connected in a series through the carrier plates.
To prevent short circuits between cells, it is important that the various plates be electrically insulated from each other, and that there is no communication of electrolyte solution directly between cells. At the same time, the plates must be mechanically supported in their relative positions during normal and shock conditions.